The avermectins are a unique collection of naturally occurring macrocyclic lactones containing an .alpha.-L-oleandrosyl-.alpha.-L-oleandrose disaccharide appended to the C.sub.13 -hydroxyl group of the aglycone unit, and exhibit anthelmintic and insecticidal properties. Since the introduction and expanded use of `abamectin` for the control of a variety of agricultural pests and the subsequent commercialization of `ivermectin` in the animal health area including the use of `MECTIZAN` for the control of riverblindness in humans a large number of avermectin derivatives have been synthesized seeking potential increases in the spectrum of parasite control in plants, animals and humans. Among these new analogues is 4"-epi-acetylamino-4"-deoxy-5-oximinoavermectin B.sub.1. This derivative has attracted attention for the control of internal parasites in companion animals and its efficient preparation in large quantity is commercially important. This compound is described in U.S. Pat. Nos. 5,015,630 and 5,023,241 being prepared by treatment of the 5-oxo compound with hydroxylamine in the presence of base which produced mainly the 3-hydroxylamino-5-oximino analog as a by-product.